1. Field of the Invention
This invention relates to an automatic focus detecting device, and more particularly to a device in which a distance measuring light is projected onto an object, a reflection resulting from the projected light is received by a light receiving or photo-sensitive element having two divided light receiving areas, the focus of a photo-taking lens is determined, and a signal is produced for controlling the focus on the basis of photo-electric signals generated by the light receiving areas.
2. Description of the Prior Art
FIG. 1 of the accompanying drawings shows a typical example of the conventional automatic focus detecting device for an imaging optical system. Here, a light projecting element LT projects a light spot onto an object OB. A light receiving photo-sensitive element PD composed of two divided photo-sensitive areas PA and PB receives a reflection of the projected light spot. The distance to the object OB or the focus of the imaging optical system is detected on the basis of the position of the reflected light on the element PD. More specifically stated with reference to FIG. 1, let us assume that the light projecting element LT projects a light spot onto an object OB1 which is located at a distance S1 and that the projected light spot impinges on and is reflected by the object OB1 to form a reflected light spot image just at a mid point between the photo-sensitive areas PA and PB of the light receiving element. Then, for an object located at another point S2 which is farther than the point S1, the reflected spot image is formed at a point on the light receiving element PD deviating from the mid point and toward the photo-sensitive area PA upward in the direction of arrow V as shown in FIG. 1. The extent of the deviation increases as the distance between the points S1 and S2 increases.
For another object OB3 which is located nearer to the apparatus than the point S1, the reflection is formed at a point on the light receiving element PD away from the mid point toward the other photo-sensitive area PB and down in the direction of arrow V as shown in FIG. 1. Accordingly, the current distance to an object can be found by detecting the position of the reflected light spot image on the light receiving element PD.
More specifically, the photo-sensitive areas PA and PB produce respective outputs whose values correspond to the quantities of light striking them. Therefore, the position at which the reflected light spot image is formed can be found by comparing the outputs of the photo-sensitive areas PA and PB of the light receiving element.
Furthermore, with an imaging optical system L arranged to form an image of an object on a predetermined focal plane FM as shown in FIG. 1, the focal point of the imaging optical system is adjusted according to the distance to the object detected as mentioned. For this purpose, the light receiving element PD is moved toward either one of the photo-sensitive areas PD and PB whichever is producing a larger output than the other in the direction of arrow V. Then, the imaging optical system L is shifted in the direction of arrow H along its optical axis X in association with the movement of the light receiving photo-sensitive element PD to make the imaging optical system L come to an in-focus position when the reflected light spot image arrives at the mid point between the photo-sensitive areas PA and PB. In other words, the optical system is in focus when the difference between the output of the photo-sensitive areas PS and the other area PB is zero. A near-focus condition (a condition in which the focal point of the imaging optical system is in front of the predetermined focal plane) prevails when the output of the photo-sensitive area PB is larger than the photo-sensitive area PA. A far-focus state (a condition in which the focal point of the imaging optical system is behind the predetermined focal plane) occurs when the output of the photo-sensitive area PA is larger than that of the other area PB. If the system is in a near-focus state, the imaging optical system L is shifted toward the predetermined focal plane FM, or to the right along the arrow H. The optical system L is shifted in the reverse direction, or to the left, along arrow H relative to the predetermined focal plane, in response to a far-focus condition. Performing this operation either manually or automatically allows the imaging optical system to be brought into focus.
As is well known, a device of this type which determines a near-focus, in-focus or far-focus condition, cannot detect the focus accurately before the integrated value of the output of the light receiving photo-sensitive element reaches a given level. For example, in the device described above, the moment the reflected light spot just impinges upon the light receiving element PD, both the outputs of the photo-sensitive elements PA and PB are nearly at noise level. Therefore, initially, it is hardly possible to find the point where the light spot image is formed. After integrating the output of the light receiving photo-sensitive element, the ratio of the signal S to a noise level N, i.e. S/N ratio, increases as the signal level increases. Eventually it becomes possible to compare the output levels of the photo-sensitive areas PA and PB with each other for accurate detection of the focal point. In other words, in order that the focal point detection be accurately accomplished, the reflected light spot must be continuously applied to the light receiving photo-sensitive element over a given length of time to permit the received light signal to be integrated sufficiently.
Therefore, automatic focus detecting devices of this kind have been arranged to project the light spot image continuously over a predetermined period of time and to detect the focal point by comparing the outputs of the light receiving element after the light quantities received have been sufficiently integrated.
However, the light intensity of the reflection upon the light receiving element varies to a great extent according to the distance and the reflection factor of the object. If the length of time for projecting the projection light spot image is fixed, the light spot has to be projected until the end of the fixed time even after the integrated value of the output of the light receiving element has already reached a focus detectable level.
This not only wastes electric energy but also results in an unnecessarily long time for detecting the focus. For a compact camera whose size does not permit the use of a high capacity power source and which must detect the focus at high speed to allow the photographer to seize a momentary picture taking opportunity, this has been quite a serious problem.